Mammary infections are initiated when bacteria penetrate a gland via the teat canal and multiply at first in the secretion from where they may eventually invade the mammary tissue. The infections frequently are of chronic nature; the disease often has a subclinical course and clinical flare-ups do occur. Bacterial species and strains endowed with potent toxins cause clinical symptoms more oftenly (1).
Several distinct protective mechanisms of acquired immunity are known to take part in the Gram-positive bacterium--ruminant host interaction; e.g. antitoxin antibody for example induced by a vaccine in which a toxoid is incorporated, attenuates injury caused by bacteria which produce this toxin and aids in preserving the functional integrity of other mechanisms of immunity; the beneficial role of opsonins in the host interaction with Gram-positive bacteria is also firmly established (6, p 627-665) and compositions that induce opsonic immunity contribute relative protection. While the value of the many "conventional" immunities, individually or in combinations, has been documented, the sum of their activities has never been satisfactory when the effect is measured in terms of elimination of existing or challenging infections from the mammary gland.
In vivo killing and elimination of Gram-positive bacteria requires phagocytosis and essential components to be involved in this process (e.g. phagocytes, complement and opsonins) are notoriously absent from the secretion of healthy mammary glands of ruminants or are present in minimal concentrations which do not provide for a major effector capacity. Such effector capacity is necessary whenever bacterial growth escapes other bacteriostatic or bactericidal controls. An efficaceous vaccine, apart from inducing various humoral and cellular immunities, must provide for induction of acquired mammary hypersensitivity (AMH) that permits the immune-mediated recruitment of the essential components whenever anergic immunities fail to contain bacterial growth or to eliminate infections. AMH is an integral part of the immune system of the mammary gland in ruminants and provides for the recruitment function. Indications are that lymphocytes present in mammary secretions and in tissues of specifically sensitized animals, are key mediators of AMH, be it in complex interaction with other cell types and with humoral factors (2,3,7). Homologous antigens can locally elicit recruitment in sensitized animals and mammary functions are normalized shortly after disappearance of the eliciting stimulus.
It therefore has been hypothesized that, if by vaccination, ruminants were made hypersensitive to bacteria, the presence of homologous bacteria in the mammary gland would trigger the recruitment reaction which would be beneficial, particularly in animals having also the other specific immunities such as antitoxic and opsonic immunity.
So far diverse mastitis vaccines and vaccination procedures have been applied or examined in order to control infections by Gram-positive bacteria. Use has been made of diverse bacterial antigen preparations, such as bacterins, cell-lysates, somatic components, toxoids or combinations of these or live attenuated bacteria or inactivated whole bacterial cultures; they have yielded, in some instances, a relative protection, measurable usually in terms of lower frequency and/or lower intensity of clinical disease symptoms rather than in terms of elimination of established or challenging infections and colonizations.